Packaging container packaging laminate and a method of producing a packaging container

ABSTRACT

A packaging container for a pressurized, liquid food, the packaging container being formed from a packaging laminate which includes at least one core layer of paper or paperboard displaying one major fibre orientation in a first direction thereof. According to the invention, said core layer is pre-strained in a second direction which is substantially at right angles to said first direction, with a view to reducing or eliminating expansion in the finished, filled packaging container during storage thereof. The present invention also encompasses a packaging laminate for the packaging container, as well as a method of producing the packaging container.

TECHNICAL FIELD

The present invention relates to a packaging container for apressurised, liquid food, e.g. beer, carbonated soft drinks, carbonatedwater or sparkling wine, the packaging container being formed from apackaging laminate which includes at least one core layer of paper orpaperboard displaying one major fibre orientation in a first directionthereof. The present invention also relates to the packaging laminatefor the packaging container, as well as a method of producing thepackaging container.

BACKGROUND ART AND PROBLEMS

Packaging containers commercially available on the market which areunder continuous inner excess pressure because they are filled with apressurised liquid food such as beer, carbonated soft drinks, carbonatedwater or sparkling wine today consist of containers produced from glass,plastic or metal.

Attempts have been made to manufacture such containers from a packaginglaminate comprising core layers of paper and paperboard, as well as gasbarrier layers and liquid barrier layers. However, in such contexts oneproblem occurs which may be traced to the stretching or straining of thefibres in the paper or paperboard layer (the core layer). The reason forthis is that, on the production of the paper or paperboard layer in apaper or paperboard machine, the fibre network is stretched in themachine direction of the paper or paperboard machine, while acorresponding stretching in the transverse direction does not takeplace. Further, the fibres are substantially oriented in the machinedirection of the paper or paperboard web as a result of the forming ofthe web when a stock jet of a cellulose stock is directed at the wirewhich runs in the machine direction.

When a tubular packaging container for a pressurised, liquid food is tobe produced from a packaging laminate which includes a core layer ofsuch paper or paperboard, the paper or paperboard is suitably utilisedsuch that the machine direction of the paper or paperboard coincideswith a tangent to the circumference of the packaging container, i.e. thepaper or paperboard is disposed such that the fibres are principallyoriented in a direction around the circular circumference of thecontainer. The reason for this is that the paper or paperboard displaysthe greatest strength in this direction, because of the fibreorientation, at the same time as the greatest strains in the material,because of the inner excess pressure, occur in this direction. However,strains will naturally also occur as a result of the inner excesspressure in the axial direction of the container, which implies that thepaper or paperboard material will expand (“creep”) in the axialdirection of the container so that the container increases in lengthafter having stood for some time on the shelf. This is particularlybecause the fibres initially, as per the above, do not display anappreciable stretching in the transverse direction of the paper orpaperboard, i.e. in the axial direction of the container. If, instead,the packaging laminate were to be turned through 90°, this problem wouldnaturally instead occur in the tangential direction of the container, inwhich event the container would swell and there would at the same timebe a risk of failure.

WO 99/59882 discloses a packaging laminate which is formed with several,e.g. four, layers of paper or paperboard. Moreover, the risk of failurehas been reduced in that the longitudinal joints for these differentlayers have been disposed in different positions around thecircumference of the container. This formation of the packaging laminateand container is not, however, commercially viable because of theextreme costs which are incurred because of the large thickness—or inother words the large quantity—of paperboard which is consumed andbecause of the complex process for realising longitudinal joints indifferent positions around the circumference of the container.

OUTLINE OF THE INVENTION

The present invention has for its object to obviate the above-outlinedproblem structure and, more specifically, to propose a packagingcontainer for a pressurised, liquid food, the packaging container beingsimple—and thereby economical—to manufacture, which does not requirelarge quantities of paper or paperboard and which nevertheless isconfigurationally stable on storage thereof with contents of apressurised, liquid food. The present invention further has for itsobject to propose a packaging laminate for a packaging containeraccording to the present invention, as well as to propose a method ofproducing a packaging container according to the present invention.

These and other objects are attained by means of the packagingcontainer, the packaging laminate, as well as the method of producingthe packaging container as these are defined in the appended claims.

The inventive concept is to take out in advance the major proportion ofthe expansion or creep of the packaging laminate, more specifically itscore layer of paper or paperboard, which occurs when this is subjectedto a continuous excess pressure. Hereby, the packaging laminate can, inthe finished packaging container, be exposed to a relatively largecontinuous loading despite a relatively slight material quantity in thecore layer, because a displacement of the creep curve of the core layertakes place. In particular, it is intended to carry out a pre-expansionor pre-straining in the transverse direction of the paper or paperboard,i.e. a direction which is substantially at right angles to the majorfibre orientation of the paper or the paperboard, normally a directionwhich is at right angles to the machine direction. This is realised inthat at least the core layer of the packaging laminate for the packagingcontainer is pre-strained in a direction (hereafter referred to as thesecond direction) which is at right angles to the direction of the majorfibre orientation (hereafter referred to as the first direction), at apoint in time prior to the closure of the packaging container.

The pre-straining according to the present invention may be carried outon the core layer already before this is laminated together with otherlayers in the packaging laminate. Alternatively, the pre-straining maybe carried out on the packaging laminate before this is reformed into acontainer body, or the pre-straining may also be carried out on thepackaging container in connection with this being filled with thepressurised, liquid food.

The core layer displays, after the pre-straining, a longitudinaldimension (L_(exp)) in the second direction, this longitudinal direction(L_(exp)) being about 1–6%, preferably 1–5% greater than thecorresponding longitudinal direction (L₀) in the core layer in the nonexpanded or strained state. This implies in other words that a dimensionbetween two points in the core layer, these two points being oriented inthe second direction in relation to one another, increases by 1–6%,preferably 1–5% when the pre-straining is carried out.

An absolute upper limit for the pre-straining naturally consists of thestrain at failure limit of the core layer or the packaging laminate, inits second direction. Another method of expressing the preferred levelof pre-straining is, therefore, to relate to the failure limit of thecore layer or the laminate, e.g. that the pre-straining is carried outwith at least 20% of the strain to failure limit, but at most 95%,preferably at most 90% and even more preferably at most 85% of thestrain to failure limit. For a paper or paperboard layer of the typeutilised here, the strain to failure limit is 1.5–2.5% in the machinedirection and 5–6% in the transverse direction.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND DETAILED DESCRIPTIONOF THE INVENTION

The present invention will now be described in greater detailhereinbelow, with reference to the accompanying Drawings, in which:

FIG. 1 shows a cross section through a first embodiment of a packaginglaminate according to the present invention;

FIG. 2 shows a cross section through a second embodiment of a packaginglaminate according to the present invention;

FIG. 3 shows a first embodiment of a packaging container according tothe present invention, seen from the side;

FIG. 4 shows a second embodiment of a packaging container according tothe present invention, seen from the side;

FIG. 5 shows a reference curve of expansion against time in a nonpre-strained core layer of paperboard which has been subjected to aloading in its transverse direction;

FIG. 6 shows a curve of expansion against time for a 1% pre-strainedcore layer of paperboard which has been subjected to a loading in itstransverse direction;

FIG. 7 shows a curve of expansion against time for a 2% pre-strainedcore layer of paperboard which has been subjected to a loading in itstransverse direction;

FIG. 8 shows a curve of expansion against time for a 3% pre-strainedcore layer of paperboard which has been subjected to a loading in itstransverse direction; and

FIG. 9 shows a curve of expansion against time for a 4% pre-strainedcore layer of paperboard which has been subjected to a loading in itstransverse direction.

FIG. 1 shows a first embodiment of a packaging laminate 10 according tothe present invention. This displays two core layers 11 a and 11 b ofpaper or paperboard. A total grammage of the paper layer in the laminateis normally 200–400 g/m², preferably 200–350 g/m² and even morepreferably 200–300 g/m², suitably distributed approximately equally onthe two core layers 11 a and 11 b. The core layers 11 a and 11 b arefurther suitably continuously joined together throughout substantiallyall of there mutually facing surfaces, by means of a layer of layer ofadhesive 12 which, for example, may consist of starch or polyethylene.On the one side of the laminate which is intended to constitute theoutside of the packaging container which is produced from the laminate,there is disposed a thin layer 13 of a thermoplastic material,preferably polypropylene. On the opposing side of the core layers 11 a,11 b, there are provided layers comprising, on the one hand, an adhesivelayer 14 of for example, adhesive thermoplastic, preferably polyethyleneor polypropylene, and, on the other hand, a barrier layer 15 as well asa thermoplastic coating 16 which is disposed outside the barrier layerand preferably consists of polypropylene, the core layer beingcontinuously covered by said layers throughout all of its surface. Theterm barrier layer 15 is here taken to signify a layer which principallyconstitutes a barrier against oxygen gas. One preferred barrier layerconsists of a metal foil layer, preferably an aluminium foil layer.

FIG. 2 shows a second embodiment of a packaging laminate 10 a accordingto the present invention which in all respects corresponds to thatillustrated in FIG. 1, but which, instead of outermost layers ofpolypropylene, displays double outer layers 17, 18 a, and 17 b, 18 b,respectively, of polyethylene and polyethylene terephthalate,respectively, the polyethylene terephthalate layer 18 a, b, constitutingthe two outermost layers in the packaging laminate. On the inside of thelaminate, the polyethylene layer 17 b suitably consist of low densitypolyethylene (LDPE).

In addition to the embodiments illustrated in FIG. 1 and FIG. 2, thepackaging laminate may naturally be built up in a large number of otherways, e.g. with other types of gas barrier layers and liquid barrierlayers. The laminate may also be formed such that it only displays onecore layer of paper or paperboard, or possibly three or more layers,this latter being, however, less preferred from the economic view point.When more than one core layer exists, at least two of these, and evenmore preferably all of these core layers of paper or paperboard aresuitably pre-strained according to the present invention.

FIG. 3 shows a first embodiment of a packaging container 20 according tothe present invention. This displays a container body 22 which consistsof a circular-cylindrical tubular body of circular cross section andprovided with a durable longitudinal joint 27, preferably made bythermosealing of the thermoplastic layers in the laminate. A lid 23 anda bottom 24 are joined together with the container body 22 by durablejoints 25, preferably made by thermosealing. The lid and bottom shown inthe Figure should only be viewed as illustrations in principle, and theactual formation and design of the lid and/or bottom may be put intoeffect in a number of different ways, including numerous variations ofopening arrangements. Further, the machine direction MD for the corelayer in the packaging container/packaging laminate has been indicatedby an arrow in the Figure. This machine direction corresponds to themajor fibre orientation and, in this context, is also designated thefirst direction. Correspondingly, the transverse direction CD of thecore layer has been marked by an arrow, and this transverse directionsubstantially coincides with a generatrix of the container body and, inthis context, is designated the second direction. The outer dimensionsof the packaging container 20 correspond typically to, for example, acontainer for beer, conventional dimensions of a beer can of metal whichtypically accommodates 33 and 50 cl.

FIG. 4 shows a second embodiment of the packaging container 21 accordingto the invention, which in all respects corresponds to the packagingcontainer 20 apart from the fact that the container body 26 displays aconically tapering configuration with circular cross section.

In the production of the packaging container or packaging laminateaccording to the present invention, the pre-straining of the corelayer/core layers is put into effect according to a first embodiment inthat a movable web of said core layer (paper or paperboard layer), thisweb displaying one major fibre orientation in its direction of movementor machine direction, is pre-strained in the second direction whichconstitutes the transverse direction of the web. More specifically, thismay, for instance, be realised in that both longitudinal edges of theweb are fixedly held by gripping members, a tensile loading beingapplied on the web in its transverse direction with the aid of a drawingdevice connected to the gripping members. Possibly, a minorpre-straining may also be put into effect in the machine direction. Itis also conceivable, instead of or in combination with a tensileloading, to execute a compressive loading on the one side of the web, atthe same time as both longitudinal edges of the web are fixedly held bythe gripping members. Such a compressive loading could be put intoeffect by means of a gas pressure which is applied on the one side ofthe web.

According to a second embodiment of the present invention, thepre-straining takes place by loading of the finished packaging laminateor on two or more layers thereof which included at least one core layerof the packaging laminate. This may be put into effect on a movable webof laminate in a manner that is fully analogous with that described inthe foregoing, or batchwise on one larger or smaller piece of packaginglaminate at a time.

According to a third embodiment of the present invention, thepre-straining is carried into effect in that the reformed container bodyis pressure-loaded interiorly with a pressurising medium before or whilethe container body is filled with the food. In such instance, it ispreferred, but not necessary, that said pressurising medium consist ofthe pressurised, liquid food proper. Alternatively, the pressurisingmedium may consist of another liquid or of a gas, e.g. air. Thepressurisation is preferably carried into effect at a pressure of 1–12bar and even more preferably 3–8 bar during a requisite period of timefor the pre-straining to take place. When the pressurising agentconsists of the liquid food, the pressure in the container body isthereafter reduced before the container body is closed. In another case,the pressure is released wholly, whereafter the container is filled withthe liquid food and closed.

According to one variation of the present invention, the loading,regardless of embodiment, may be put into effect at elevatedtemperature, above room temperature, and/or at elevated moisture contentin the paper or the paperboard, with a view to increasing the effect ofthe loading.

EXAMPLES

FIG. 5 shows a reference curve of expansion against time for a nonpre-strained Kraft paper consisting of two layers of 145 g/m² each, theKraft paper being subjected to a loading in the transverse direction of27.1 kNm/kg at 80% relative humidity. After roughly 10 minutes, thepaper had been stretched in the transverse direction an average ofaround 4.5%. (The different curves show the results from severaldifferent experiments under the same conditions.)

FIGS. 6–9 show curves for corresponding experiments carried out on thesame type of Kraft paper which had been pre-strained by 1, 2, 3 and 4%,respectively, prior to the experiments. It may be read off from thecurves that the stretching of the material on loading reduces inrelation to the degree of pre-straining. At 4% pre-straining, the paperhad been stretched in the transverse direction, after roughly 10minutes' loading of 27.1 kNm/kg at 80% relative humidity by an averageof about 3.2%.

The present invention is not restricted to the embodiments described inthe foregoing, but may be varied without departing from the scope of theappended claims.

1. A packaging container for a pressurized, liquid food, the packagingcontainer being formed from a packaging laminate which includes at leastone core layer of paper or paperboard, said core layer displaying onemajor fibre orientation in a first direction thereof, wherein said corelayer is pre-strained in a second direction which is substantially atright angles to said first direction, the pre-strained core layerdisplaying a longitudinal dimension in the second direction that isroughly 1–6% larger than the corresponding longitudinal dimension of thecore layer in the non pre-strained state.
 2. The packaging container asclaimed in claim 1, wherein it includes a tubular container body whichdisplays a cylindrical or conically tapering configuration with circularcross section, said second direction substantially coinciding with ageneratrix to the container body.
 3. The packaging container as claimedin claim 1, wherein the pre-strained core layer displays a longitudinaldimension in the second direction that is 1–5% larger than thecorresponding longitudinal dimension of the core layer in the nonpre-strained state.
 4. The packaging container as claimed in claim 1,wherein said packaging laminate includes two or more core layers ofpaper or paperboard, preferably at least two of them and even morepreferably all of these core layers of paper or paperboard beingpre-strained in a second direction which is substantially at rightangles to said first direction in each respective core layer.
 5. Thepackaging container as claimed in claim 1, wherein said packaginglaminate also includes a gas barrier layer.
 6. A packaging laminate fora packaging container as claimed in claim 1, the packaging laminateincluding at least one core layer of paper or paperboard, said corelayer displaying one major fibre orientation in a first directionthereof, wherein said core layer is pre-strained in a second directionwhich is substantially at right angles to said first direction, thepre-strained core layer displaying a longitudinal dimension in thesecond direction that is roughly 1–6% larger than the correspondinglongitudinal dimension of the core layer in the non pre-strained state.7. The packaging laminate as claimed in claim 6, wherein thepre-strained core layer displays a longitudinal dimension in the seconddirection that is 1–5% larger than the corresponding longitudinaldimension of the core layer in the non pre-strained state.
 8. Thepackaging laminate as claimed in claim 6, wherein it includes two ormore core layers of paper or paperboard, preferably at least two ofthese and even more preferably all of these core layers of paper orpaperboard being pre-strained in a second direction which issubstantially at right angles to said first direction in each respectivecore layer.
 9. The packaging laminate as claimed in claim 6, wherein italso includes a gas barrier layer.
 10. A method of producing a packagingcontainer for a pressurized, liquid food, a packaging laminate beingproduced, said packaging laminate including at least one core of paperor paperboard, said core layer displaying one major fibre orientation ina first direction thereof, a sheet-shaped blank of said packaginglaminate being reformed into a container body which is thereafter filledwith said pressurized, liquid food and closed, wherein said core layeris pre-strained in a second direction which is substantially at rightangles to said first direction at a point in time either before orduring the production of said packaging laminate or before the reformingof said sheet-shaped blank into a container body, or prior to or whilethe container body is filled with the food, the core layer beingpre-strained so that a longitudinal dimension of the core layer in thesecond direction increases by roughly 1–6% relative to a correspondinglongitudinal dimension of the core layer prior to being pre-strained.11. The method as claimed in claim 10, wherein a movable web of saidcore layer or a movable web of said packaging laminate, said webdisplaying one major fibre orientation in its direction of movement ormachine direction, is pre-strained in the second direction whichconsists of the transverse direction of the web, preferably in that bothlongitudinal edges of the web are fixedly held at the same time as theweb is subjected to a tensile loading in the transverse direction, sothat a longitudinal dimension of the web in said transverse directionincreases by 1–5% in relation to a corresponding longitudinal dimensionprior to said tensile loading.
 12. The method as claimed in claim 10,wherein said container body is compression loaded interiorly by apressuring medium before or while the container body is filled with thefood, so that a longitudinal dimension of the container body increasesby 1–5% in relation to a corresponding longitudinal dimension prior tosaid compression loading, said longitudinal dimension being oriented insaid second direction which preferably coincides with a generatrix tothe container body.
 13. The method as claimed in claim 12, wherein saidpressurizing medium consists of a gaseous pressurizing medium which ispreferably applied at a pressure of 1–12 bar and even more preferably3–8 bar.
 14. The method as claimed in claim 12, wherein saidpressurizing medium consists of said pressurized, liquid food, which ispreferably filled at a pressure of 1–12 bar and even more preferably 3–8bar during a requisite period of time for said pre-straining to takeplace, whereafter the pressure is reduced before the container body isclosed into a packaging container.
 15. The method as claimed in claim10, wherein said pre-straining is carried out at at least 20%, but atmost 95%, preferably at most 90% and even more preferably at most 85% ofa strain to failure limit for said core layer or said packaginglaminate.
 16. The method as claimed in claim 10, wherein saidpre-straining is carried out at an elevated temperature and/or atelevated moisture content in the core layer or core layers.